The thyrotropin response to the diminished serum triiodothyronine concentrations of non-thyroid illness implies a true deficiency of thyroid hormone at target tissues and the assumption that this will limit the anabolic response to nutritional support of severely ill patients, we propose to study the effects fo triiodothyronine supplementation during parenteral and enteral nutritional support. We also plan to study the relationship between thyroid hormone distribution on serum protein binding sites, particularly prealbumin, and free triiodothyronine concentration. Our purpose is to test the hypothesis that the thyroxine prealbumin complex is a preferential precursor for the extrathyroidal production of triiodothyronine. Related studies will examine the effect of triiodothyronine on recovery from fasting in experimental animals. We also plan to study the relationship between the thyroxine prealbumin complex and triiodothyronine generation in the vitamin A deficient rat in which the concentration of the thyroxine prealbumin complex is diminished. This proposal is consistent with evidence that endocrine control and particularly the activating deiodination of thyroxine will prove to be a critical aspect of nutritional adaptation.